Eichlseder method (U.S. Pat. No. 4,880,587) and apparatus (U.S. Pat. No. 4,738,613) teaches a way to form the centerhole in an injection molded optical disk by selective mechanical retention via undercuts molded into portions of the sprue element, such that when a certain sequence of ejector motions is employed, the sprue is separated away from the molded disk, before either are ejected out of the optical disk moldset, by a tearing action of the annular film gate connecting the sprue element to the molded disk. Although other ways of forming the centerhole in an optical disk are well known and described in other prior art patents, the Eichlseder teachings are employed in commercially available Krauss Maffei compact disk (CD) molds, which are believed to be the single most successful brand of CD molds, with over 300 such molds installed worldwide.
Applicants U.S. Pat. No. 5,068,065 issued Nov. 26, 1991 and incorporated herein by reference teaches an alternative to Eichlseder, wherein a "hollow sprue" geometry is employed for faster cooling rates and greater ease of filling and packing. This patent also teaches alternative ways to form the centerhole, such as a "molded in" centerhole formed when the annular gate aperture between sprue and disk is mechanically closed before the plastic therein is solidified. Although Applicants' invention may have technical merits in newly-designed CD molds, the great commercial success of Eichlseder is duly noted. Retrofits of the hollow sprue type into such existing CD molds of the Eichlseder type may not be practical or economically feasible. Therefore, Applicants have set about to find ways of improving the existing Eichlseder CD molds without "major surgery".
When Eichlseder CD molds were first commercialized, the state-of-the-arts molding cycle time was 9-10 seconds. Cycle time has been reduced now to the 5 second range. Therefore, it is not surprising that whereas the Eichlseder sprue can run problem-free at the original longer cycle times, problems occur at total cycle times of 5 seconds or less, wherein the cooling time portion is less than or equal to 3.0 seconds. The onset of failure is that, when the Eichlseder sprue is not yet sufficiently solidified, the mechanical forces of the ejector pin (which strips the sprue away from the ejector core's undercut) will exceed the "hot strength" of the B side portion of the sprue, and plastic deformation occurs. If the plastic deformation of the sprue only results in some small bulging, this is not a functional problem. But if the ejector pin forces are sufficiently greater than the hot strength of the sprue resisting these forces, the ejector pin (moving from the movable B side of the mold towards the stationary A side of the mold) actually tears right through the sprue, leaving the sprue's flange separated from the conical "funnel" upstream portion. Once the sprue is torn in two parts as described herein, the sprue cannot cleanly be ejected and the sprue "part verification" circuit will trip an alarm and automatic molding cycle interruption will occur, resulting in downtime. In the Eichlseder molds, this onset of failure is noted when the cooling time becomes less than or equal to 3.0 seconds, which tends to correspond to a total cycle time of 5.0 to 5.5 seconds total cycle time. To get below this threshold value in cycle time requires some improvements to be made.
Cecchi (U.S. Pat. No. 4,069,003) teaches the use of a fluted sprue bushing to form a concentric pattern of rib elements molded into the circumferential surfaces of the upstream portion of the sprue (defined herein as that conically shaped tapered portion starting with the junction with the nozzle tip and ending with the plane of the parting line, wherein lies the flange and optical disk). This ribbing is claimed to provide for faster cycling for those cases where the sprue is the slowest portion of the injection molded shot (consisting of the molding product plus the sprue and runner system), in which case if the sprue is slower to become sufficiently rigid for ejection and transfer out of the mold of the molded product, then such an approach can be useful.
However, in the case of optical disk injection molding, a mere combination of Eichlseder with Cecchi is not useful, because Cecchi teaches placing his ribs onto the portion of the sprue located between the injection molding machine's nozzle tip and the mold cavity wherein the molded article is formed. Cecchi ribs made optimally large enough to provide some structural rigidity to the sprue while hot would also cause disruption of the injected molten plastic as it flows between the injection molding machine's nozzle tip and the mold cavity, thus creating into the disk a "starburst" radial flow pattern. These non-uniform meltfront flow rates (with faster flow at the relatively less obstructed fluted rib crosssections having a relatively more open aparture to flow, and the more constrictive adjacent root-diameter conical sections having a greater restriction or impedence to flow) will result in cosmetic flowlines, as well as a "starburst" radial pattern of unacceptably high birefringence (molded-in stress, seen under polarized light) which would be a functional problem to the proper playing of compact disk, since the laser beam will be deflected by this non-uniform pattern of molded in stress.
Most importantly, Cecchi teachings apply to only to rigidifying the sprue only between the injection molding machine's nozzle tip and the mold cavity. However, it is the movement of the ejector pin which causes plastic deformation on the B or moveable ("displaceable") half side of the sprue and not on the A or stationary half side of the sprue (when viewed with respect to the parting line as the division between A and B side of the optical disk mold). In other words, it is the defficiencies of hot strength on the B side, not the A side, which are the current limitations to the Eichlseder sprue. Cecchi contributes nothing to the B side problems of the sprue; his drawings do not even show that portion of the sprue.
Even if it were permissable to apply, by hindsight, to employ Cecchi ribs onto the B side of the sprue to thereby rigidify the Eichlseder sprue against the compressive deformation of the ejector pin, Applicants have found, only certain geometries of these B side ribs avoid the disruption of flowfront problems mentioned earlier. Refer to details of the Applicants specification, herein.
Unlike Cecchi, Marsh (U.S. Pat. No. 2,306,316) does deal with problems related to the B side or ejector side of the sprue but merely teaches how to improve cooling rate of that portion of the sprue, by inserting a cooling finger element actively cooled by circulation of heat transfer fluid. Such a hollowed-out base of the B side of the sprue by itself contributes nothing to the Eichlseder problem. The inside and outside surfaces formed in the Marsh design are shown to be of a smooth conical taper (no ribs), as is the case with all the other known prior art, particularly those many optical disk-specific prior art references cited in Applicants' U.S. Pat. No. 5,068,065.